During the past few years we have developed a procedure whereby a bacterium can be infected with a phage and most, if not all, of the phage DNA can be isolated free of bacterial DNA. The types of molecules and the number of each type can be determined at any time after infection by direct electron microscopic observation. This technique has been used successfullly with E. coli phage Lambda to answer several basic questions about Lambda DNA replication. In this project the technique will be used to examine replication and recombination of Lambda DNA in phage-infected cells. Of immediate interest are (1) the determination of the replication origin of rolling circles and (2) cataloguing the structures present in cells infected with recombination-deficient phage. Many novel types of molecules have already been observed; their structures will be analyzed and special attention will be paid to those that are recombination intermediates. The role of oligomeric circles will also be studied. We will also examine the intracellular molecules isolated from po1A- and lig- bacteria infected with Lambda red and gam, since in these cases there are replication defects of an unknown nature.